xrender.pc (/usr/portage/x11-libs/cairo/files/xrender.pc) should really be somewhere in the x11 package somewhere with the rest of xrender
It's actually installed by xorg-x11 (~arch) and xfree-4.3.0-r6 (which will be removed from hard-mask shortly, pending bug #1343 and bug #21450). So as soon as they hit stable, cairo can stop doing that. By the way, I hope you're hacking that xrender.pc to have the proper version and so forth based on what's installed.
I agree the xfree and xorg-x11 should provide xrender.pc. See bug #40400. AFAIK xfree-4.3.0-r6 is still masked in package.mask. When this is unmasked I'll remove the xrender.pc from cairo. I've no plans to update cairo's xrender.pc to report the version: it's non-trivial to do reliably (probably best plan is to find highest ver of ${R}/usr/X11R6/lib/X11/Xrender.so.*, can't call XRenderQueryVersion because X not nescessarily running when we install the package) but IMNSHO it's simply not worth the hassle given that 4.3.0-r6 will be available 'soon'. I'm open to disagreements :-)
x11-libs/cairo no longer installs xrender.pc.